Field of the Invention
The present invention relates to a wiper blade for a motor vehicle or aircraft.
Wiper blades for motor vehicles or aircraft are exposed to severe mechanical stresses, on the one hand from dust, insect residues or other hard dirt ingredients that make the glass surface to be cleaned rough, and on the other by the tilting stress, which changes continually during the wiping operation and is permanent in the position of repose as a consequence of the persistent contact pressure against the glass. Besides the mechanical stresses there are also chemical or environmental factors, for instance from ozone, oil, and alcohol and/or detergents in the windshield washing fluid. Wiper blades with suitably adapted profiles (or cross-sections) are extruded, as a rule continuously, from cross-linkable (or vulcanizable) polymer mixtures, which are cross-linked after extrusion and are cut to the requisite length. The profiles have a head (also called a base part or spine), with which the wiper blade is secured by the claws in the windshield wiper blade holder bracket system. Recesses for one or more spring rails of elastic metal or plastic located in the head are intended to stabilize the wiper blade and bring about the most uniform possible contact pressure over the entire length of the wiper blade.
On its side toward the glass, the head changes into a tilting rib, also known for short as a rib, neck, hinge, or changeover rib, and connects the head to the lip, which as the actual functional part of the wiper blade keeps the glass free of water when the windshield wiper is in operation. The tilting rib is the mechanically most severely stressed part of the wiper blade. Depending on its height and on the design of the opposed faces of the head and lip, the lip with its main cross-sectional axis can deviate in the position of repose by up to about 45.degree. from the vertical on the glass. In the operating state the lip trails behind the head, thus forcing a tilting motion each time there is a change of direction. The main cross-sectional axis accordingly changes its position in operation constantly by up to approximately .+-.45.degree..
The materials of which wiper rubber is made are natural or synthetic polymers or copolymers with double bonds, which are cross-linked for instance with organic peroxides and/or sulfur and/or certain metal oxides. Wiper blades of the prior art, with all the parts mentioned, may all be made of a single material. While this is favorable for production purposes, it does not suitably meet the various demands made of the different parts of the wiper blade. For instance, if the material is relatively soft and elastic, it meets important demands made of the tilting rib but is less suitable for the head, because the head during operation is not infrequently twisted out of the claws that hold the wiper blade in the metal holder bracket. Conversely, if the material is stiffer and less elastic, then while it is well-suited for the head it i not so appropriate in view of the tilting stress to which the tilting rib is constantly exposed.
Wiper blades of only a single material, namely natural rubber or a mixture of natural rubber and chloroprene rubber, or a rubber of the diene type, such as butadiene rubber and styrene-butadiene rubber, are mentioned in German Patent DE-C2 35 27 093 as being part of the prior art. Natural rubber and other kinds of rubber of the diene type also include olefin double bonds and are therefore vulnerable to ozone. The tendency to ozonolysis can be repressed by means of certain additives. By "halogenation", for instance treatment with a hypochlorite solution, the surface of the wiper blade can be surface-hardened. This is advantageous for the lip, because it increases the abrasion resistance of the lip. However, compromises must be made, because overly extensive halogenation of the surface, which is desirable for the sake of abrasion resistance, can make the lip overly nonconforming, so that it no longer adapts well enough to the glass, which is usually curved. For a tilting rib of natural rubber, or natural rubber and other kinds of rubber of the diene type, however, halogenation is harmful because it promotes a persistent deformation under compressive stress. The wiper blade is exposed to such a stress both in the position of repose and in operation, because it is pressed constantly against the glass. In addition, the halogenation favors the formation of cracks under the influence of sunlight, which for the mechanically highly stressed tilting rib during wiping, as a result of the continual tilting motions, is especially undesirable.
The wiper blade according to the invention defined by DE-C2 5 27 093 substantially comprises only a single material, namely EPDM rubber, which is a copolymer of ethylene, propylene and a diene, that is cross-linked with peroxides and is largely resistant to ozone. Only on the lower end of the lip, toward the glass, and in the upper peripheral zones of the head does the wiper blade have small segments of smaller amounts of diene rubber. EPDM rubber often also includes an oil, which improves flexibility at low temperatures. This oil tends to migrate to the periphery (or sweat out) and particularly in relatively long phases of repose, the head and lip, which touch one another in the state of repose, tend to stick together as a result. Moreover, over the short or long term, the oil also migrates into the diene rubber and to the surface thereof that touches the glass. Dust and dirt that stick to the oil then cause smears and streaks during wiping. In addition, EPDM rubber is not resistant to chemicals. Windshield wiper cleaning fluids that contain alcohol thus gradually impair the physical properties of the rubber. Finally, EPDM has a high coefficient of friction, which makes it difficult to insert especially internal spring strips, and makes it necessary to coat the surface of the spring strips or to use lubricant aids, such as molybdenum sulfide.
Wiper blades of different, coextruded materials have also been disclosed in which, unlike those in DE-C2 35 27 093, the various materials are used in comparable amounts. In European Patent Disclosure EP-A1 0 625 452, a wiper blade is disclosed that comprises at least two different materials. In one of the embodiments described (FIG. 1), the head and the tilting rib comprise the same material, while the lip of a different material includes a reinforcement into which the narrow tilting rib runs out. This embodiment has the disadvantage described above, which is that the material does not fully meet the various demands made of the head and the rib.